The present invention relates to the thermistor element which is, for example, suitable for use in temperature measurement with high accuracy and method of manufacturing the same.
Thermistor has the physical property such that its electrical resistivity widely varies with change in temperature. Especially, a NTC (negative temperature coefficient) thermistor whose electrical resistivity decreases with increase in temperature has a wide range of applications. One usage of such thermistor is temperature measurement.
FIG. 1 shows prior art thermistor element for temperature measurement typified by U.S. Pat. No. 3,815,074. Reference numeral 1 represents a thermistor chip of sinter powdery material. Thick film electrodes 2 and 3 are formed by firing heat-resisting conductive painted films containing nobel metal such as Ag, PdAg, Pt and Au to fix it to the both surfaces of the thermistor chip 1. Lead wires 4 and 5 are connected at their base portions to the electrodes 2 and 3 by firing the heat-resisting conductive material 6. The thermistor chip 1, electrodes 2 and 3 and base portions of the lead wires 4 and 5 thus assembled are embedded in a glass body 7.
However, in the prior art thermistor element, it is hard to obtain ohmic contact between the thermistor chip 1 and the thick film electrode 2 or 3 owing to frits contained in the heat-resisting conductive paint. For the reason, voltage dependancy or current dependancy characteristics of the variation in resistance values in such thermistor element becomes unstable, resulted from errors of characteristics such as B factor, resistance value, in certain circumstances.
Moreover, a large amount of noble metal such as Au or Pt is needed for forming the thick film electrodes 2,3 on the thermistor chip 1, which leads to high manufacturing cost. Furthermore, it is feared that the property of the thermistor chip 1 is deteriorated by baking the heat-resisting conductive paint, upon forming the thick film electrodes 2,3 or upon connecting the lead wires 4,5 to the electrodes 2,3 respectively. In that case, characteristic of the thermistor element is liable to be scattered.
It is an object of the present invention to overcome the above disadvantages of the prior art by providing a thermistor element with excellent ohmic contact, whereby to improve current (or voltage) dependancy characteristics of the variation in resistance values thereof.
It is another object of the present invention to provide a thermistor element with a high yield rate and with low manufacturing cost.
According to the present invention, there is provided a thermistor element comprising, a sintered thermistor chip in wafer form with two principal opposite surfaces, thin film electrodes evaporated on respective principal surfaces of the thermistor chip, lead wires with one of the ends being respectively adhered to the thin film electrodes with heat-resisting conductive material, and glass body for sealing the thermistor chip, the thin film electrodes and the adhered ends of the lead wires.